bundle exec rails g migration add_parent_folder_ref_to_folders parent_folder:references

# This creates a change method, but I rename it to up and add a down method, to allow rollback
# I also manually added foreign_key for integrity (default to false)
class AddParentFolderRefToFolders < ActiveRecord::Migration
  def up
    add_reference :folders, :parent_folder, index: true, foreign_key: true
  end

  def down
    remove_reference :folders, :parent_folder, index: true, foreign_key: true
  end
end

# Run bundle exec rake db:migrate